The cardiovascular system’s base job is to supply the required volume of blood to peripheral tissue to ensure adequate perfusion under a range of different conditions.

In essence, this means that whether one is exercising, sleeping or sitting etcetera, the cardiovascular system has to supply a sufficient quantity of blood to all the present tissues and organs so they can function properly.

In order for this to be achieved, certain factors have to remain in a state of homeostasis;

Myocardial networks

Neural and hormonal influence

The blood volume in circulation

and

The compliance and resistance of the peripheral vascular system

The cardiac cycle has three defined phases:

LV contraction

LV relaxation

LV filling

*Whilst similar phases are in occurrence on the right side of the heart, the left side is the one that is described

In greater detail however, it can be split into 7 phases:

Atrial Systole

Isovolumetric Contraction

Rapid Ejection

Reduced Ejection

Isovolumetric Relaxation

Rapid Filling

Reduced Filling

Atrial Systole

MV, TV open

AV, PV closed

At rest: atrial contraction responsible for 10% of LV filling

During exercise: 4x increase in LV filling

Isovolumetric Contraction

MV, TV, AV, PV closed

LV volume is fixed

Rapid Ejection

AV, PV open

MV, TV closed

LV pressure peaks, then begins to descend

LVEDV quickly declines

Reduced Ejection

AV, PV open

MV, TV closed

Reduced ejection rate due to ventricular repolarisation

Isovolumetric Relaxation

MV, TV, AV, PV closed

Aortic pressure rises slightly, giving dicrotic notch

No change in blood volume

EDV 50ml approx

Lusitopy of LV- pressure drops until it’s lower than LA, then MV opens, leading to:

Rapid Filling

MV, TV open

AV, PV closed

Accounts for most of LV filling with active diastolic relaxation

Reduced Filling

MV, TV open

AV, PV closed

LV becomes 90% full

Filling virtually stops when equilibrium between atria and ventricles is reached

Cardiac Output (CO) is the volume of blood pumped by the heart in one minute.

It is given by the equation:

CO (mL/min) = SV (mL/beat) * HR (BPM)

So if the average human has a HR of 70bpm and an average SV of 70mL/beat, we have a cardiac output of:

70 * 70 = CO = 4900mL/min

Stroke Volume is the volume of blood ejected from the ventricle in a single beat and is controlled by

Preload

Afterload

Inotropy

The relationship can be seen here:

Preload denotes the initial stretching of cardiomyocytes prior to contraction, so the ‘load’ is the stretch of the cardiac muscle at the end of diastole.

An increase in preload increases EDV and subsequently, an increased SV.

Afterload is the term for the ‘load’ the heart must contract again to eject blood; the lower the afterload, the greater the volume of blood ejected with each contraction. An increased afterload increases ESV and reduced SV